1. Field of the Invention
The present invention relates generally to the field of integrated circuits and subsystems. Specifically, the present invention relates to an active balun circuit for single-ended to differential RF signal conversion with enhanced common-mode rejection.
2. Description of the Related Art
A differential-pair amplifier can be used in integrated circuits as an active balun when single-ended to differential conversion is needed, e.g., the radio-frequency (RF) and local oscillator (LO) ports of balanced mixers. The challenge is to maintain its phase and amplitude balance at high frequency when parasitic effects degrade its performance. Several techniques have been reported to improve the phase and amplitude balance as well as the bandwidth.
However, for integrated circuits used in wireless communications, narrow band response is a preferred feature because it helps reject unwanted signals. For example, a narrow band balun, i.e., a passive device with distributed electrical constants for coupling a balanced system or device to an unbalanced system or device, placed between a low-noise amplifier (LNA) and mixer will help reject out-of-band interference signals and the balun placed between the LO and mixer will reduce harmonics from LO. If these interference signals or harmonics enter the mixer, they may create higher order intermodulation products that can block the wanted signal. When broadband is not a design target, narrow band techniques can be used to achieve better performance in a specific frequency range.
On the other hand, an active balun has limited dynamic range when compared to its passive counterpart. In order to increase its linearity under limited supply voltage, large transistors with large current bias have to be used. This increases its parasitic capacitance and affects signal balance due to common-mode leakage or signal at high frequency.